Gamma correction for adjustable light source

ABSTRACT

The present invention provides a projection apparatus comprising: a light source, a light source control unit for controlling the output of the light source; at least one spatial light modulator for modulating the illumination light from the light source by multiple pixel elements; and an optical system for projecting, onto a screen, the illumination light deflected by the spatial light modulator, wherein: the light source control unit 1) modulates the output of the illumination light from the light source during a modulation period of the spatial light modulator, and 2) non-linearly controls the gray scale of an image projected onto the screen.

This application is a Non-provisional Application claiming a Prioritydate of Sep. 6, 2007 based on a previously filed Provisional Application60/967,953 and a Non-provisional patent application Ser. No. 11/121,543filed on May 4, 2005 issued into U.S. Pat. No. 7,268,932 and anotherNon-provisional application Ser. No. 10/698,620 filed on Nov. 1, 2003.The application Ser. No. 11/121,543 is a Continuation In Part (CIP)Application of three previously filed Applications. These threeApplications are Ser. No. 10/698,620 filed on Nov. 1, 2003 nowabandoned, Ser. No. 10/699,140 filed on Nov. 1, 2003 now issued intoU.S. Pat. No. 6,862,127, and Ser. No. 10/699,143 filed on Nov. 1, 2003now issued into U.S. Pat. No. 6,903,860 by the Applicant of this PatentApplications. The disclosures made in these Patent Applications arehereby incorporated by reference in this Patent Application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to image projection apparatusesimplemented with mirror devices functioning as a spatial light modulator(SLM). The present invention relate more particularly to an imageprojection apparatus implemented with mirror devices functioning as SLMto receive illumination light from a light source that includes a lightcontrol unit to modulate the illumination light to control the grayscale of an image projection with non-linear control processes.

2. Description of the Related Art

Even though there have been significant advances in recent years in thetechnology of implementing Electromechanical micro-mirror devices asspatial light modulators, there are still limitations and difficultieswhen these are employed to display high quality images. Specifically,when an image is digitally controlled, the image quality is adverselyaffected because the image is not displayed with a sufficient number ofgray scales.

Electromechanical micro-mirror devices have drawn considerable interestbecause of their application as spatial light modulators (SLMs). Aspatial light modulator requires a relatively large number ofmicro-mirror devices. In general, the number of devices required rangesfrom 60,000 to several million for each SLM. Refer to FIG. 1A for adigital video system 1 as disclosed in relevant U.S. Pat. No. 5,214,420,which includes a display screen 2. A light source 10 is used to generatelight energy to illuminate display screen 2. Light 9 is furtherconcentrated and directed toward lens 12 by mirror 11. Lens 12, 13, and14 serve a combined function as a beam columnator to direct light 9 intoa column of light 8. A spatial light modulator 15 is controlled by acomputer through data transmitted over data cable 18 to selectivelyredirect a portion of the light from path 7 toward lens 5 to display onscreen 2. The SLM 15 has a surface 16 that includes switchablereflective elements, e.g., micro-mirror devices 32 with elements 17, 27,37, and 47 as reflective elements attached to a hinge 30, as shown inFIG. 1B. When element 17 is in one position, a portion of the light frompath 7 is redirected along path 6 to lens 5 where it is enlarged orspread along path 4 to impinge the display screen 2 so as to form anilluminated pixel 3. When element 17 is in another position, light isnot redirected toward display screen 2 and hence pixel 3 would be dark.

The on-and-off states of a micro-mirror control scheme, such as thatimplemented in the U.S. Pat. No. 5,214,420 and by most conventionaldisplay systems, limits image display quality. This is because theapplication of a conventional control circuit limits the gray scale (PWMbetween ON and OFF states) by the LSB (least significant bit, or theleast pulse width). Due to the ON-OFF states implemented in conventionalsystems, there is no way to provide a pulse width shorter than the LSB.The least brightness, which determines the gray scale, is the lightreflected during the least pulse width. A limited gray scale leads tolower image quality.

In FIG. 1C, a circuit diagram of a control circuit for a micro-mirroraccording to U.S. Pat. No. 5,285,407 is presented. The control circuitincludes memory cell 32. Various transistors are referred to as “M*”where * designates a transistor number and each transistor is aninsulated gate field effect transistor. Transistors M5, and M7 arep-channel transistors; transistors, M6, M8, and M9 are n-channeltransistors. The capacitances, C1 and C2, represent the capacitive loadspresented to memory cell 32. Memory cell 32 includes an access switchtransistor M9 and a latch 32 a, which is the basis of the static randomaccess switch memory (SRAM) design. All access transistors M9 in a rowreceive a DATA signal from a different bit-line 31 a. The particularmemory cell 32 to be written is accessed by turning on the appropriaterow select transistor M9, using the ROW signal functioning as aword-line. Latch 32 a is formed from two cross-coupled inverters, M5/M6and M7/M8, which permit two stable states. State 1 is Node A high andNode B low and state 2 is Node A low and Node B high.

The switching of the dual states, as illustrated by the control circuit,controls the micro-mirrors to position either at an ON or an OFF angularorientation, as shown in FIG. 1A. The brightness, i.e., the gray scalesof display for a digitally control image system, is determined by thelength of time the micro-mirror stays at an ON position. The length oftime a micro-mirror is at an ON position is, in turn, controlled by amultiple bit word. FIG. 1D shows the resultant binary time intervalswhen the device is controlled by a four-bit word. As shown in FIG. 1D,the time durations have relative values of 1, 2, 4, and 8 that in turndefine the relative brightness for each of the four bits where 1 is forthe least significant bit and 8 is for the most significant bit.According to the control mechanism as shown, the minimum controllabledifferences between gray scales for showing different brightness is abrightness represented by the “least significant bit” that keeps themicro-mirror at an ON position.

For example, assuming n bits of gray scales, the frame time is dividedinto 2^(n)−1 equal time periods. For a 16.7 milliseconds frame periodand n-bit intensity values, the time period is 16.7/(2^(n)−1)milliseconds

Having established these time slices for controlling the length of timefor displaying each pixel in each frame, the pixel intensities aredetermined by the number of time slices represented by each bit.Specifically, a display of a black pixel is represented by 0 timeslices. The intensity level represented by the LSB is 1 time slice, andmaximum brightness is 2^(n)−1 time slices. The number time slices that amicro mirror is controlled to operate at an On-state in a frame perioddetermines a specifically quantified light intensity of each pixelcorresponding to the micromirror reflecting a modulated light to thatpixel. Thus, during a frame period, each pixel corresponding to amodulated micromirror controlled by a control word with a quantifiedvalue of more than 0 is operated at an on state for the number of timeslices that correspond to the quantified value represented by thecontrol word. The viewer's eye integrates the pixel brightness so thatthe image appears the same as if it were generated with analog levels oflight.

For addressing deformable mirror devices, a pulse width modulator (PWM)receives the data formatted into “bit-planes”. Each bit-planecorresponds to a bit weight of the intensity value. Thus, if eachpixel's intensity is represented by an n-bit value, each frame of datahas n bit-planes. Each bit-plane has a 0 or 1 value for each displayelement. In the example described in the preceding paragraphs, eachbit-plane is separately loaded during a frame. The display elements areaddressed according to their associated bit-plane values. For example,the bit-plane representing the LSBs of each pixel is displayed for 1time period.

Projection apparatuses such as described above generally use a lightsource such as a high-pressure mercury lamp, a xenon lamp, or similarkinds of light sources. However, these types of light sources performpoorly in high-speed switching that alternates between the ON and OFFstates. Accordingly, a light source is usually controlled tocontinuously operate in an ON state during the entire length of timewhen the projection apparatus is in operation. The light source that iscontinuously turned on generates a large amount of heat and wastes lightand electricity.

There is also an increasing demand that projection apparatuses projectimages at a higher level of gray scale (gradation). Accordingly, aspatial light modulator has to be controlled to enable a projectionapparatus to project images at a higher level of gray scale. However,the achievable improvement of the gray scale performance would be verylimited if the improvements are to be achieved only through the controlof the spatial light modulator limited. Some of the attempts to furtherimprove the quality of image display have been disclosed in manyPatents, such as U.S. Pat. Nos. 5,214,420, 5,285,407, and publishedPatent Applications. However, the disclosures including those includedin the Information Disclosure Statement (IDS) have not providedeffective solution to resolve the above discussed difficulties andlimitations.

Recent developments in image display technology; a so-called γcorrection is performed when the cameras capture the images. The γcorrection is carried out in order to take into account the projectioncharacteristics of the CRT devices for displaying the image as the TVimages projected from the CRT devices.

Therefore, the signal voltages E applied in the CRT devices forprojecting the TV images have a functional relationship with the imageprojection output L represented by L=E^(γ). In other words, therelationship when multiplied by a non-linear γ correction is non-linear.Also, in order to reduce the cost to the TV receivers as consumers, thisγ correction is performed on the transmission side when image data isgenerated.

In contrast, unlike the CRT devices, the display characteristic of theprojection apparatuses using micro-mirror devices as described above islinear. Accordingly, it is necessary to perform a reverse correction onbroadcasted image signals in order to cancel the γ correction performedon the transmission side.

An example of the γ correction performed on the reception side is one inwhich a prescribed mathematical operation is performed on the input dataitself. However, this mathematical operation for the γ correction iscomplicated because its use of a logarithm function. Also, a largerscale operation circuit is required, which to increases the productioncosts of projection apparatuses.

It is also possible to adapt a conversion technology using a lookuptable or the like, thereby avoiding such mathematical operations.However, in order to attain an acceptable accuracy of operation (i.e.,conversion accuracy), the gray scale accuracy of the input data has tobe increased (in other words, the number of bits has to be increased)before conversion, which forces the lookup table or the like to occupy agreater volume of memory, which increases the production costs ofprojection apparatuses.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide an improved imagedisplay apparatus implemented with a new technology to achieveadjustable and high performance display images with higher resolution ofgray scales. The improved apparatuses and control methods are notlimited by the display gray scale performance of the conventionalspatial light modulator such that the above-discussed difficulties andlimitations are resolved.

It is another aspect of the present invention to provide an improvedimage display apparatus implemented with a spatial light modulator witha higher resolution of gray scales to permit a gamma correction in aprojection apparatus without making complicated configurationmodifications or increasing the production costs of the image displayapparatuses.

A first exemplary embodiment of the present invention provides aprojection apparatus that includes: a light source, a light sourcecontrol unit for controlling the output light, at least one spatiallight modulator for modulating illumination light from the light sourceby pixel elements, and an optical system for projecting, onto a screen,the illumination light deflected by the spatial light modulator, whereinthe light source control unit modulates the output of the illuminationlight from the light source during a modulation period of the spatiallight modulator, and non-linearly controls the gray scale of an imageprojected to the screen.

A second exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment withthe light source control unit modulates output of the illumination lightby making an intensity of the illumination light variable.

A third exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit modulates output of theillumination light by making an emission interval cycle of pulseemission of the light source variable.

A fourth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit modulates output of theillumination light by making an emission interval cycle of pulseemission of the light source constant and by making an emission pulsetime variable.

A fifth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit modulates output of theillumination light by making an emission interval cycle and an emissionpulse time of pulse emission of the light source variable.

A sixth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit modulates output of theillumination light by making at least one of an emission interval cycle,an emission pulse time, and an emission pulse intensity of pulseemission of the light source variable.

A seventh exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit modulates output of theillumination light by input image data.

An eighth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source control unit increases maximum brightness ofthe light source by making an emission interval cycle of the pulseemission variable within a particular time of one frame when output ofthe illumination light is modulated by making an emission interval cycleof pulse emission of the light source variable.

A ninth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the spatial light modulator comprises a micromirror device inwhich a plurality of mirror elements for deflecting light from the lightsource are arranged.

A tenth exemplary embodiment of the present invention provides theprojection apparatus according to the first exemplary embodiment,wherein the light source is a light emitting diode (LED) or a laserdevice.

An eleventh exemplary embodiment of the present invention provides aprojection apparatus, includes at least one light source provided foreach of colors of illumination light, a light source control unit forcontrolling output of the light source, at least one spatial lightmodulator for modulating the illumination light from the light source bya plurality of pixel elements, and an optical system for projecting, toa screen, the illumination light modulated by the spatial lightmodulator, wherein the light source control unit modulates a projectedimage by changing an emission energy of cyclic pulse emission for eachcolor of the illumination light emitted from the light source.

A twelfth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes emission pulse width ofthe cyclic pulse emission for each color of the illumination lightemitted from the light source.

A thirteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes an emission pulse cycle ofthe cyclic pulse emission for each color of the illumination lightemitted from the light source.

A fourteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes an emission pulseintensity of the cyclic pulse emission for each color of theillumination light emitted from the light source.

A fifteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes an emission interval cycleand emission pulse width of the cyclic pulse emission for each color ofthe illumination light emitted from the light source.

A sixteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes an emission interval cycleand an emission pulse intensity of the cyclic pulse emission for eachcolor of the illumination light emitted from the light source.

A seventeenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit changes emission pulse width andan emission pulse intensity of the cyclic pulse emission for each colorof the illumination light emitted from the light source.

An eighteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the spatial light modulator comprises a micromirror device inwhich a plurality of mirror elements for deflecting light from the lightsource are arranged.

A nineteenth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source is a light emitting diode (LED) or a laserdevice.

A twentieth exemplary embodiment of the present invention provides theprojection apparatus according to the eleventh exemplary embodiment,wherein the light source control unit demodulates output of theillumination light by input image data.

A twenty-first exemplary embodiment of the present invention provides aprojection apparatus, includes at least one light source provided foreach of colors of illumination light, a light source control unit forcontrolling output of the light source, at least one micromirror devicein which a plurality of mirror elements for deflecting the illuminationlight from the light source are arranged, a micromirror device controlunit for controlling the micromirror device, and a projection opticalsystem for projecting, to a screen, the deflected illumination lightfrom the micromirror device, wherein the light source control unitperforms a modulation control of an accumulated maximum light intensityin a display period of one frame corresponding to the light source ofeach color of the illumination light.

A twenty-second exemplary embodiment of the present invention providesthe projection apparatus according to the twentieth-first exemplaryembodiment, wherein the micromirror device control unit causes themirror element corresponding to pixel data of maximum brightness in theframe data for each color to be in a state of continuously leading theillumination light to the projection optical system during a displayterm of the one frame, and the light source control unit performs amodulation control so that a desired output light intensitycorresponding to the pixel data of the maximum brightness is obtained.

A twenty-third exemplary embodiment of the present invention providesthe projection apparatus according to the twentieth-first exemplaryembodiment, wherein the light source is a light emitting diode (LED) ora laser device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail below with reference to thefollowing figures.

FIG. 1A is a conceptual diagram of a projection apparatus according toconventional technology;

FIG. 1B is a conceptual diagram of the mirror element of a projectionapparatus according to conventional technology;

FIG. 1C is a conceptual diagram showing of the drive circuit of a mirrorelement of a projection apparatus according to conventional technology;

FIG. 1D is a conceptual diagram of image data used in the projectionapparatus according to conventional technology;

FIG. 1E is a side cross sectional view for illustrating the etendue byexemplifying the case of using a discharge lamp light source andprojecting an image by way of an optical device;

FIG. 2 is a diagram showing the relationship between the numericalaperture NA1 of an illumination light path, the numerical aperture NA2of a projection light path and the tilt angle α of a mirror;

FIG. 3A is a cross-sectional view illustrating an example of a mirrordevice according to the preferred embodiment 1-1;

FIG. 3B is a cross-sectional view of a part along line B-B′ in anexample of a mirror element in the mirror device shown in FIG. 3Aaccording to the preferred embodiment 1-1;

FIG. 3C is a cross-sectional view of a part along line A-A′ in anexample of the mirror element in the mirror device shown in FIG. 3Aaccording to the preferred embedment 1-1;

FIG. 4 shows another configuration of an electrode;

FIG. 5A is a side view diagram showing an example of a deflection statewhen the mirror is ON;

FIG. 5B is a side view diagram showing an example of a deflection statewhen the mirror is OFF;

FIG. 5C is another side view diagram showing an example of thedeflection state when the mirror is OFF;

FIG. 6 is a conceptual diagram showing a projection apparatus accordingto the preferred embodiment of the present invention;

FIG. 7 is a conceptual diagram showing a single-panel projectionapparatus according to another preferred embodiment of the presentinvention;

FIG. 8A is a block diagram showing a control unit for a single-panelprojection apparatus according to the preferred embodiment of thepresent invention;

FIG. 8B is a block diagram showing the control unit of a multi-panelprojection apparatus according to the preferred embodiment of thepresent invention;

FIG. 9 is a conceptual diagram showing the light source drive circuit ofa projection apparatus according to the preferred embodiment of thepresent invention;

FIG. 10 is a chart showing the relationship between the applied currentand the intensity of light emission drive circuit according to theembodiment of the present invention;

FIG. 11 is a conceptual diagram showing the layout of the internalconfiguration of a spatial light modulator according to the embodimentof the present invention;

FIG. 12 is a cross-sectional diagram of an individual pixel unitconstituting a spatial light modulator according to the preferredembodiment of the present invention;

FIG. 13 is a conceptual diagram showing an individual pixel unitconstituting a spatial light modulator according to the embodiment ofthe present invention;

FIG. 14 is a chart illustrating the principle of a γ correction of videoimage data

FIG. 15 is a chart illustrating the principle of a γ correction bycontrolling the emission intensity of a light source performed in aprojection apparatus according to the embodiment of the presentinvention;

FIG. 16 is a chart illustrating the conversion of binary data tonon-binary data performed in a projection apparatus according to theembodiment of the present invention;

FIG. 17 is a chart illustrating the conversion of binary data tonon-binary data performed in a projection apparatus according to theembodiment of the present invention;

FIG. 18 is a chart illustrating the conversion of binary data tonon-binary data performed in a projection apparatus according to theembodiment of the present invention;

FIG. 19 is a chart illustrating the conversion of binary data tonon-binary data performed in a projection apparatus according to theembodiment of the present invention;

FIG. 20 is a chart showing a γ correction of the brightness input ofeight-bit non-binary data. The figure illustrates the implementation infour stages, performed in a projection apparatus according to theembodiment of the present invention;

FIG. 21 is a chart showing a γ correction of the brightness input ofeight-bit non-binary data. The illustrates a modification of theimplementation in four stages, performed in a projection apparatusaccording to the embodiment of the present invention;

FIG. 22 is a chart showing a γ correction by means of an intermittentpulse emission performed in a projection apparatus according to theembodiment of the present invention;

FIG. 23A is a chart showing a γ correction by means of an intermittentpulse emission, thereby increasing the correction of the lowerbrightness side, performed in a projection apparatus according to theembodiment of the present invention;

FIG. 23B is a chart showing the γ correction curve performing a γcorrection by means of a light source pulse pattern illustrated in FIG.23A, which increases on the correction of the lower brightness side;

FIG. 24A is a chart illustrating a γ correction with consideration forhuman vision by means of an intermittent pulse emission in a projectionapparatus according to the embodiment of the present invention;

FIG. 24B is a chart illustrating a γ correction with consideration forhuman vision by means of the light source pulse pattern exemplified inFIG. 24A;

FIG. 25 is a chart illustrating gray scale control by keeping a mirrorin a constant ON state and controlling the intensity of light emission,which is performed in a multi-panel projection apparatus according tothe embodiment of the present invention;

FIG. 26 is a chart illustrating gray scale control by keeping a mirrorin a constant ON state and controlling the pulse emission of a lightsource, which is performed in a multi-panel projection apparatusaccording to the embodiment of the present invention;

FIG. 27 is a chart illustrating gray scale control by keeping a mirrorin a constant ON state and controlling the intensity of emission of alight source, which is performed in a single-panel projection apparatusaccording to the embodiment of the present invention;

FIG. 28 is a chart illustrating gray scale control by keeping a mirrorin a constant ON state and controlling the pulse emission of a lightsource, which is performed in a single-panel projection apparatusaccording to the embodiment of the present invention;

FIG. 29 is a diagram showing the principle of increasing the range ofgray scale control by a combination of the ON/OFF control of a mirrorand the emission intensity control of a light source, which is performedin a projection apparatus according to the embodiment of the presentinvention; and

FIG. 30 is a chart illustrating the prevention of color break by acombination of the ON/OFF control of a mirror and the oscillationcontrol of the mirror performed in a projection apparatus according tothe embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will beexplained by referring to the <<Disclosure Contents>> provided below.

Disclosure Content 1 Preferred Embodiment 1-1

The preferred embodiment 1-1 of the present invention relates to amirror device that arranges deflectable mirrors and to a method forregulating the deflection angle of a mirror.

FIG. 1E cross sectional view for showing the etendue of a lightprojecting system using a discharge lamp as the light source to projectan image through an image by way of an optical device.

Outline of the Device

The first is a description of a mirror device.

Image projection apparatuses implemented with a spatial light modulator(SLM), such as a transmissive liquid crystal, a reflective liquidcrystal, a mirror array and other similar image modulation devices, arewidely known.

A spatial light modulator is formed as a two-dimensional array ofoptical elements, ranging from tens of thousands to millions ofminiature modulation elements, with individual elements enlarged anddisplayed as the individual pixels corresponding to an image to bedisplayed onto a screen by way of a projection lens.

The spatial light modulators generally used for projection apparatusesprimarily include two types, i.e., a liquid crystal device formodulating the polarizing direction of incident light by sealing aliquid crystal between transparent substrates and providing them with apotential, and a mirror device deflecting miniature micro electromechanical systems (MEMS) mirrors with electrostatic force andcontrolling the reflecting direction of illumination light.

One embodiment of the above described mirror device is disclosed in U.S.Pat. No. 4,229,732, in which a drive circuit using MOSFET anddeflectable metallic mirrors are formed on a semiconductor wafersubstrate. The mirror deflects to different angles according to theelectrostatic force supplied from the drive circuit and is capable ofchanging the reflecting direction of the incident light.

Meanwhile, U.S. Pat. No. 4,662,746 has disclosed an embodiment in whichone or two elastic hinges retain a mirror. If the mirror is retained byone elastic hinge, the elastic hinge functions as bending spring. If themirror is retained by two elastic hinges, they function as torsionsprings to incline the mirror, thereby the deflecting the reflectingdirection of incident light.

As described above, the ON-and-OFF states of a micromirror controlscheme as that implemented in U.S. Pat. No. 5,214,420 and by mostconventional display systems limits display quality. Specifically,conventional control circuits limits the gray scale (PWM between ON andOFF states) since it is limited by the LSB (least significant bit, orthe least pulse width). Due to the ON-OFF states implemented inconventional systems, there is no way to provide a shorter pulse widththan the LSB. The lowest brightness, which determines the gray scale, isthe light reflected during the least pulse width. The limited gray scaleleads lower image quality.

Specifically, FIG. 1C exemplifies a conventional control circuit for amicromirror according to U.S. Pat. No. 5,285,407. The control circuitincludes memory cell 32. Various transistors are referred to as “M*”where * designates a transistor number and each transistor is aninsulated gate field effect transistor. Transistors M5 and M7 arep-channel transistors; transistors, M6, M8, and M9 are n-channeltransistors. The capacitances, C1 and C2, represent the capacitive loadsof the memory cell 32. Memory cell 32 includes an access switchtransistor M9 and a latch 32 a, which is the basis of the static randomaccess switch memory (SRAM) design. All access transistors M9 in a rowreceive a DATA signal from a different bit-line 31 a. The particularmemory cell 32 is accessed and written by turning on the appropriate rowselect transistor M9, using the ROW signal functioning as a wordline.Latch 32 a is formed from two cross-coupled inverters, M5/M6 and M7/M8,which permit two stable states. State 1 is Node A high and Node B lowand state 2 is Node A low and Node B high.

The mirror driven by a drive electrode abuts a landing electrodestructured differently from the drive electrode, thereby a prescribedtilt angle is maintained. A “landing chip”, which possesses a springproperty, is formed on the point of contact between the landingelectrode and the mirror. This configuration enhances the deflection ofthe mirror to the reverse direction upon a change in the control. Theparts forming the landing chip and the landing electrode are maintainedat the same potential so that contact will not cause a shorting or othersimilar disruption.

Outline of PWM Control

The following description explains the pulse-width modulation (PWM)control.

As described above and shown in FIG. 1A, an operation for switching themirror by the control circuit deflects the micromirrors in either an ONor an OFF angular orientation. The brightness, i.e., the gray scales ofdisplay for a digitally controlled image system is determined by thelength of time the micromirror stays at an ON position. The length oftime a micromirror is controlled at an ON position is controlled by amultiple bit word. FIG. 1D shows the “binary time intervals” when themicromirror is controlled by a four-bit word. As in FIG. 1D, the timedurations have relative values of 1, 2, 4, 8 that, in turn, define therelative brightness for each of the four bits where 1 is the leastsignificant bit and 8 is the most significant bit. According to thecontrol mechanism as shown, the minimum controllable differences betweengray scales for showing different brightness is the brightnessrepresented by the “least significant bit” that can maintain themicromirror at an ON position.

In a simple exemplary display system operated with a n bits brightnesscontrol signal for controlling the gray scales, the frame time isdivided into 2^(n)−1 equal time slices. For a 16.7 milliseconds frameperiod and n-bit intensity values, the time slice is 16.7/(2^(n)−1)milliseconds.

Having established these time slices for controlling the length of timefor displaying each pixel in each frame, the pixel intensities aredetermined by the number of time slices represented by each bit.Specifically, a display of a black pixel is represented by 0 timeslices. The intensity level represented by the LSB is 1 time slice, andmaximum brightness is 2^(n)−1 time slices. The number time slices that amicro mirror is controlled to operate at an On-state in a frame perioddetermines a specifically quantified light intensity of each pixelcorresponding to the micromirror reflecting a modulated light to thatpixel. Thus, during a frame period, each pixel corresponding to amodulated micromirror controlled by a control word with a quantifiedvalue of more than 0 is operated at an on state for the number of timeslices that correspond to the quantified value represented by thecontrol word. The viewer's eye integrates the pixels' brightness so thatthe image appears the same as if it were generated with analog levels oflight.

For addressing deformable mirror devices, a pulse width modulator (PWM)receives the data formatted into “bit-planes”. Each bit-planecorresponds to a bit weight of the intensity value. Thus, if eachpixel's intensity is represented by an n-bit value, each frame of datahas n bit-planes. Each bit-plane has a 0 or 1 value for each displayelement. In the example described in the preceding paragraphs, eachbit-plane is separately loaded during a frame. The display elements areaddressed according to their associated bit-plane values. For example,the bit-plane representing the LSBs of each pixel is displayed for 1time slice.

Outlines of Mirror Size and Resolution

The following description explains the size of a mirror and theresolution.

The size of a mirror for constituting such a mirror device is between 4μm and 20 μm on each side. The mirrors are placed on a semiconductorwafer substrate in such a manner as to minimize the gap between adjacentmirrors. Smaller gaps reduce random and interfering reflection lightsfrom the gap to prevent such reflections from degrading the contrast ofthe displayed images. The mirror device is formed a substrate thatincludes an appropriate number of mirror elements. Each mirror elementis applied to modulate a corresponding image display element known as apixel. The appropriate number of image display elements is determinedaccording to image display standards in compliance to the resolution ofa display specified by the Video Electronics Standards Association(VESA) and to the television-broadcasting standard. For example, in thecase of configuring a mirror device in compliance with the WXGA (withthe resolution of 1280×768) as specified by VESA and in which the sizeof each mirror is 10 μm, the diagonal length of the display area will beabout 0.61 inches, thus producing a sufficiently small mirror device

Outline of Projection Apparatus

The following a description is for a projection apparatus.

The projection apparatuses using deflection-type (“deflectable”) lightmodulators are primarily categorized into two types, i.e., asingle-panel projection apparatus comprising a single spatial lightmodulator, which spatially changes the frequency of a projection lightand displays an image in colors, and a multi-panel projection apparatuscomprising spatial light modulators, which constantly modulate theillumination light with different frequencies by means of individualspatial light modulators and displays an image in colors by synthesizingthese modulated lights.

The single-panel projection apparatus is configured as FIG. 1A describedabove.

Outline of the Introduction of Laser Light Source

The following a description is for a laser light source.

In the projection apparatus that includes a reflective spatial lightmodulator implemented with a mirror described above, there is a closerelationship between the numerical aperture (NA) NA1 of an illuminationlight path, the numerical aperture NA2 of a projection light path, andthe tilt angle α of a mirror. FIG. 2 shows the relationship betweenthem.

Assuming that the tilt angle α of a mirror 1011 is 12 degrees. When amodulated light reflected by mirror 1011 and incident to the center ofthe projection light path is set perpendicular to a device substrate1012, the illumination light is incident from a direction inclined by2α, that is, 24 degrees, relative to the perpendicular of the devicesubstrate 1012. For the light beam reflected by the mirror to be mostefficiently incident to the center of the projection lens, the numericalaperture of the projection light path should be equal to the numericalaperture of the illumination light path. If the numerical aperture ofthe projection light path is smaller than that of the illumination lightpath, the illumination light cannot be sufficiently projected into theprojection light path; however, if the numerical aperture of theprojection light path is larger that that of the illumination lightpath, the illumination light can be entirely directed; the projectionlens then becomes unnecessarily large. Further in this event, the lightfluxes of the illumination light and projection light need to be placedapart from each other because the optical members of the illuminationsystem and those of the projection system need to be physicallyseparated. Keeping the above considerations in mind, when a spatiallight modulator with the tilt angle of a mirror being 12 degrees isused, the numerical aperture (NA) NA1 of the illumination light path andthe numerical aperture NA2 of the projection light path are preferablyset as follows:NA1=NA2=sin α=sin 12°

If the F-number of the illumination light path is F1 and the F-number ofthe projection light path is F2, then the numerical aperture can beconverted into an F-number as follows:F1=F2=1/(2*NA)=1/(2*sin 12°)=2.4

In order to maximize the use of illumination light emitted from anon-coherent light source, such as a high-pressure mercury lamp or axenon lamp, which is generally used for projection apparatuses, theprojection angle of light must be maximized on the illumination lightpath side. Since the numerical aperture of the illumination light pathis determined by the tilt angle of a mirror to be used, it is clear thatthe tilt angle of the mirror needs to be large in order to increase thenumerical aperture of the illumination light path.

However, increasing of the tilt angle of the mirror requires a higherdrive voltage for driving the mirror.

Because a greater tilt angle of the mirror requires physical space toallow for the tilting of the mirror, a greater distance must existbetween the mirror and the electrode driving the mirror. Theelectrostatic force F generated between the mirror and the electrode isgiven by the following equation:F=(∈*S*V ²)/(2*d ²),

where “S” is the area size of the electrode, “V” is a voltage, “d” isthe distance between the electrode and mirror, and “c” is thepermittivity of vacuum.

The equation shows that the drive force decreases in proportion to thesecond power of the distance d between the electrode and the mirror. Itis possible to increase the drive voltage to compensate for the decreasein the drive force associated with the increase in the distance;conventionally, however, the drive voltage is about 3 to 15 volts in thedrive circuit by means of a CMOS process used for driving a mirror and,therefore, a relatively special process such as a DMOS process isrequired if a drive voltage in excess of about 15 volts is needed. Thatis not preferable cost reduction remains a consideration.

Further, in order to reduce the cost of a mirror device, it is desirableto obtain as many mirror devices as possible from a single semiconductorwafer substrate for the improvement of productivity. That is, a decreasein the size of mirror elements reduces the size of the mirror device perse. It is clear that the area size of an electrode is reduced inassociation with a decrease in the size of the mirror, which alsorequires less driving power in accordance with the above equation.

In contrast to the need to decrease the size of a mirror device, thelarger a mirror device, the brighter it can illuminate as long as aconventional lamp is used. This is because a conventional lamp with anon-directivity in its emission allows the usage efficiency of light tobe substantially reduced. This is attributable to a relationshipcommonly called etendue.

As shown in FIG. 1E above, where “y” is the size of a light source 4150,“u” is the importing angle of light on the light source side of theillumination lens 4106, “y′” is the size of the image of a light source,and “u′” is the converging angle on the image side (device 4107), therelationship between these, when the projected image is to be projectedvia the device 4107 and the projection lens, is represented by thefollowing equation:y*u=y′*u′

That is, the smaller the device onto which a light source will projectan image, the smaller the importing angle on the light source sidebecomes. This is why it is advantageous to use a laser light source,which possesses strong directivity of emission light, in order todecrease the size of the mirror device.

Outline of Resolution Limit

The following description is to explain a resolution limit.

The following discussions are based on an examination of the limit valueof the aperture ratio of a projection lens used for a projectionapparatus. The projection lens displays the image on a display surfaceby enlarging the image modulated with specific resolution and reflectedfrom a spatial light modulator. The resolution of projected image can befurther understood according to the following descriptions.

A symbol “Rp” denotes the pixel size of the spatial light modulator,“NA” represents the aperture ratio of a projection lens, “F” is an Fnumber and “λ” is the wavelength of light, the limitative “Rp” withwhich any adjacent pixels on the projection surface are separatelyobserved is derived by the following equation:Rp=0.61*λ/NA=1.22*λ*FThe table below shows the F value of a projection lens and thedeflection angle of a mirror by shrinking the mirror size that furthershortens the distance between the adjacent mirror elements. Thewavelength of light beam is designated at a value of λ=650 nmrepresenting a lowest value within the range of visible light.Meanwhile, the F value of a projection lens with the wavelengthdesignated at 700 nm is about 7% smaller than the F value for thewavelength of 650 nm.

Pixel size of mirror device F number of Deflection angle of [μm]projection lens mirror [degrees] 4 5.04 5.69 5 6.30 4.55 6 7.56 3.79 78.82 3.24 8 10.08 2.84 9 11.34 2.52 10 12.61 2.27 11 13.87 2.06

Therefore, since the difficulties related to the above describedconcerns with etendue is circumvented by using a laser light for thelight source, the F numbers of lenses for the illumination system andprojection system can be increased to the values shown in the table.Therefore, it is achievable to decrease the deflection angle of themirror element, and thereby, a smaller mirror device with a low drivevoltage can be configured.

Outline of Oscillation Control

The following a description explains the oscillation control process.

US Patent Application 20050190429 discloses another method for reducinga drive voltage. A mirror is held in a free oscillation state in theinherent oscillation frequency, and thereby, the intensity of light thatis about 25% to 37% of the emission light intensity when a mirror iscontrolled under a constant ON state can be obtained during theoscillation period of the mirror.

By controlling and operating the mirror with intermediate oscillationstate, it is no longer required to drive the mirror in high speed.Furthermore, the intermediate oscillation states enable the projectionof images with higher levels of gray scale. The mirror also has hingesthat have a low spring constant as a spring member for supporting themirror, and therefore enabling a reduction in the drive voltage.Furthermore, combining this technique with the method of decreasing thedrive voltage by decreasing the deflection angle of the mirror, asdescribed above, will increase the level of gray scale to a greaterdegree. As described above, the use of a laser light source makes itpossible to decrease the deflection angle of a mirror and also shrinkthe mirror device without decreasing the display brightness.Additionally, by implementing the above-described oscillation controlmethod enables a higher level of gray scale without causing an increasein the drive voltage.

However, if an electrode for driving a mirror and a stopper fordetermining the deflection angle of the mirror are individuallyconfigured, as in the conventional method, the problem of inefficientspace usage remains.

U.S. Pat. No. 5,583,688, US Patent 20060152690, U.S. Pat. No. 6,198,180,and U.S. Pat. No. 6,992,810 disclose configurations for determining thedeflection angle of a mirror in a conventional mirror device. Theabove-disclosed configurations however have difficulties for a person ofordinary skill in the art to increase the size of address electrodes. Inconsideration of the problems noted above, the preferred embodiment 1-1of the present invention is accordingly configured to integrate theelectrode used for driving a mirror element with a stopper used fordetermining the deflection angle of a mirror.

The following is a description, in detail, of a mirror device accordingto the present embodiment.

FIG. 3A is a top view of a mirror for illustrating the configuration ofthe mirror element of a mirror device according to the preferredembodiment 1-1. FIG. 3B is a side cross-sectional view for illustratingthe configuration of the mirror element of a mirror device according tothe preferred embodiment 1-1. FIG. 3C is another side cross-sectionalview for illustrating the configuration of the mirror element of amirror device according to the preferred embodiment of 1-1.

FIGS. 3A to 3C show a mirror element that includes a mirror 1101 havinga highly reflective top surface coated with material, such as aluminumor gold. The mirror 1101 is supported by an elastic hinge 1102 made of asilicon material, a metallic material and the like, and is placed on asubstrate 1103. The silicon material may include poly-silicon, singlecrystal silicon and amorphous silicon, while the metallic material mayinclude aluminum, titanium, and an alloy of some of these metallicmaterials, or a composite of these metals or alloys. The mirror 1101 isin the approximate shape of a square, with the length of one side, forexample, between 4 μm and 11 μm. The size of adjacent mirrors is alsobetween, for example, 4 μm and 11 μm. The deflection axis 1111 of themirror 1101 is on the diagonal line. The lower end of the elastic hinge1102 is connected to the substrate member 1103 that includes a circuitfor driving the mirror 1101. The upper end of the elastic hinge 1102 isconnected to the bottom surface of the mirror 1101. An electrode forreliably maintain electrical connectivity and an intermediate member forstrengthening the support structures and strengthening the connectionmay be placed between the elastic hinge 1102 and substrate 103, orbetween the elastic hinge 1102 and mirror 1101.

In FIGS. 3A through 3C, electrodes 1104 (i.e., 1104 a and 1104 b) usedfor driving the mirror 1101 are placed on the top surface of thesubstrate 1103 opposite to the bottom surface of the mirror 1101. Theform of the electrode 1104 may be symmetrical or asymmetrical about thedeflection axis 1111. The electrode 1104 is made of aluminum ortungsten. The present embodiment is configured such that the electrode1104 also carries out the function of a stopper for contacting andstopping the mirror from further movement when the mirror is deflectedto a maximum deflection angle. The deflection angle of the mirror is theangle determined by the aperture ratio of a projection lens thatsatisfies a theoretical resolution determined by the size of adjacentmirrors on the basis of the equation described below:Rp=0.61*λ/NA=1.22*λ*F

Alternatively, larger deflection angle may be implemented. For example,the deflection angle of the mirror 1101 may be controlled to operatebetween 10 degrees and 14 degrees relative to the horizontal state ofthe mirror 1101 or between 2 degrees and 10 degrees relative to thehorizontal state of the mirror 1101. Configuring the electrode 1104 toalso function as stopper makes it possible to maximize the electrodelayout space when shrinking the mirror element, as compared to theconventional method of placing the electrode and stopper individually.

The electrode may be formed, as shown in FIGS. 3A through 3C, as atrapezoid with the top and bottom sides approximately parallel to thedeflection axis 1111, and the sloped sides approximately parallel to thecontour line of the mirror 1101 of the mirror device. The deflectionaxis of the mirror 1101 is matched with the diagonal line of theelectrode. The electrode and stopper can usually be simultaneouslymanufactured as in the conventional method, and therefore such theelectrode and stopper may be conveniently formed during themanufacturing processes.

A difference in potentials must be generated between the mirror andelectrode to drive the mirror with electrostatic force. The presentembodiment using the electrode also as a stopper provides the surface ofthe electrode or/and the rear surface of the mirror with an insulationlayer(s) in order to prevent electrical shorting at the contact pointbetween the mirror and the electrode. Furthermore, when the surface ofthe electrode has an insulation layer, the insulation layer can coveronly the part that contacts the mirror. FIGS. 3A through 3C illustratethe surface of the electrode 1104 (i.e., 1104 a and 1104 b) with aninsulation layer 1105 (i.e., 1105 a and 1105 b). The insulation layer ismade of oxidized compound, azotized compound, and silicon or siliconcompound, e.g., SiC, SiO₂, Al₂O₃, and Si. The material and thickness ofthe insulation layer is determined so that the dielectric strengthvoltage is maintained at no less than the voltage required to drive themirror, most preferably no less than 5 volts. For example, thedielectric strength voltage may be two times the drive voltage of themirror or higher, 3 volts or higher or 10 volts or higher.

The following descriptions explain the size and shape of the electrodein one exemplary embodiment of this invention. Referring to FIG. 4 for aside cross sectional view of a mirror, where “L1” is the distancebetween the deflection axis and the edge of the electrode on the sideclosest to the deflection axis of the mirror 1101. The distance shown by“L2” is the distance between the deflection axis and the edge of theelectrode on the side farthest from the deflection axis of the mirror1101, and “d1” and “d2” are the distances between the mirror bottomsurface and the top of the electrode at its respective edges. “P1” is arepresentative point at the electrode edge on the side closest to thedeflection axis of the mirror and “P2” is a representative point at theelectrode edge on the side farthest from the deflection axis of themirror.

FIG. 4 shows an electrode formed so that d1<d2. In this configuration,the stopper determines the tilt angle of the mirror 1101 and should beplaced at the point P2 in consideration of a production variance of theelectrode height that influences the deflection angle of the mirror. Thepresent embodiment is accordingly configured to satisfy the relationshipof:d1>(L1*d2)/L2

This configuration significantly improves the efficient usage of thespace under the mirror and maintains a stable deflection angle of themirror.

In the case of configuring the electrode to provide distances d1 and d2with d1=d2, the point P2 on the corner of the electrode determining thedeflection angle of the mirror, and the configuration is determined tosatisfy the following equation:cot θ=d2/L2

The following is a description of the circuit comprisal of the mirrordevice according to the present embodiment.

The circuit comprisal of the mirror device according to the presentembodiment is illustrated in FIGS. 11 and 13, both of which aredescribed later, and therefore their descriptions are not provided here.

Referring to FIGS. 5A to 5C for showing the light reflection from themirrors. The mirror configuration and operation of mirror deflectionspresent different deflection states when different voltages are appliedto the electrodes. The light incident to the mirror 1212 is deflected ina specific direction as shown in FIGS. 5A through 5C. The following adescription outlines the natural oscillation frequency of theoscillation system of a mirror device according to the presentembodiment.

As discussed above, a reduced drive voltage when applied to a mirrorwith intermediate oscillation states can achieve a gray scale withhigher resolution. With a least significant bit (LSB) defining a minimumcontrollable light intensity in a pulse width modulator (PWM) throughmirror oscillation, the natural oscillation cycle of an oscillatingmirror supported on an elastic hinge is further described below. Thenatural oscillation cycle T of an oscillation system can be determinedas:T=2*π*√(I/K)=LSB time/X[%];where:

-   -   I is the rotation moment of an oscillation system,    -   K is the spring constant of an elastic hinge,    -   LSB time is the LSB cycle at displaying n bits, and    -   X [%] is the ratio of the light intensity obtained by one        oscillation cycle to the full-ON light intensity of the same        cycle        Note that:    -   “I” is determined by the weight of the mirror and the distance        between the center of gravity and the center of rotation;    -   “K” is determined by the thickness, width, length, and        cross-sectional shape of an elastic hinge;    -   “LSB time” is determined from one frame time, or one frame time        and the number of reproduction bits in the case of a        single-panel projection method;    -   “X” is determined as in the above description, particularly from        the F-number of a projection lens and the intensity distribution        of an illumination light.

For example, when a single-panel color sequential method is employed,the ratio of emission intensity by one oscillation is assumed to be 32%and the minimum emission intensity in a 10-bit grayscale is achievableby an oscillation, then “I” and “K” are designed so as to have a naturaloscillation cycle as follows:T=1/(60*3*2¹⁰*0.32)≈17.0 μsec.

In contrast, when a conventional PWM control is employed, the changeovertransition time t_(M) of a mirror is approximately equal to the naturaloscillation frequency of the oscillation system of the mirror.Accordingly, the LSB is required to control the mirror that the lightintensity in the interim during the changeover transition is negligible.The gray scale produced with the above-described hinge is about 8-bit,even if the LSB is set at five times the changeover transition timet_(M). In other words, compared with a conventional display system, thedisplay systems of this invention can achieve an image display with a10-bit grayscale by using the 8-bit grayscale control signals because ofthe implementation of the intermediate control states.

In another exemplary embodiment for an image display system to displayimage with a 13-bit gray scale by a single-panel projection apparatusdescribed above, the length of time represented by a LSB can becalculated as follows:LSB time=(1/60)*(1/3)*(1/2¹³)=0.68 μsec

For a display system with intermediate control state controllable toproject 38% of the ON state intensity, the oscillation cycle T is asfollows:T=0.68/38%=1.8 μsec

In contrast, when attempting to obtain an 8-bit grayscale in amulti-panel projection apparatus described above, the length of timerepresented by the LSB can be calculated as follows:LSB time=(1/60)*(1/3)*(1/2⁸)=21.7 μsec

In another exemplary embodiment, when the light intensity obtained inone cycle by controlling the mirror to operate in an intermediate stateis 20% of the ON state intensity, then the oscillation cycle T can beset as follows:T=21.7/20%=108.5 μsec.

As described above, the present embodiment is configured to set themirror to oscillate with a natural oscillation cycle of the oscillationsystem. The display system includes an elastic hinge. The naturaloscillation cycle can be set between about 1.8 μsec and 108.5 μsec withthree deflection states. The mirror element in a first deflection stateis modulated by the mirror element directed towards the projection lightpath. The mirror element in the second deflection state is controlled toreflect a light in a direction away from the projection light path. Themirror element in a third deflection state is controlled to oscillatebetween the first and second deflection states. The intermediate stateprovide a reduced amount of controllable light thus enabling the displaysystem to display images of a gray scale with higher resolution withoutincreasing the drive voltage of the mirror element. As described above,the present embodiment is configured to make the electrode also functionas a stopper for regulating the maximum deflection angle of the mirror.By configuring the electrode to sever also as a stopper increases theefficiency of space usage while shrinking the mirror element andexpanding the area of the electrode.

Disclosure Content 2

The following detail description is provided for an exemplary embodimentof the present invention by referring to the accompanying drawings.

FIG. 6 is a functional block diagram for showing the configuration of aprojection apparatus according to a preferred embodiment of the presentinvention.

FIG. 6 shows a projection apparatus 5010 according to the presentembodiment comprises a single spatial light modulator (SLM) 5100, acontrol unit 5500, a Total Internal Reflection (TIR) prism 5300, aprojection optical system 5400, and a light source optical system 5200.

The projection apparatus 5010 is a commonly referred to as asingle-panel projection apparatus comprising a single spatial lightmodulator 5100.

The projection optical system 5400 includes the spatial light modulator5100 and a TIR prism 5300 disposed on the optical axis of the projectionoptical system 5400, and the light source optical system 5200 isdisposed for projecting a light along the optical axis matches with theoptical path of the projection optical system 5400.

The TIR prism 5300 receives the incoming illumination light 5600projects from the light source optical system 5200 and directs the lightto transmit as incident light 5601 to the spatial light modulator 5100at a prescribed inclination angle. The SLM 5100 further reflects andtransmits a the reflection light 5602, towards the projection opticalsystem 5400. The projection optical system 5400 receives the light 5602reflected from the SLM 5100 onto a screen 5900 as projection light 5603.The light source optical system 5200 comprises a variable light source5210 for generating the illumination light 5600, a condenser lens 5220for focusing the illumination light 5600, a rod type condenser body5230, and a condenser lens 5240.

The variable light source 5210, condenser lens 5220, rod type condenserbody 5230, and condenser lens 5240 are placed in the aforementionedorder on the optical axis of illumination light 5600 emitted from thevariable light source 5210 and incident to the side face of the TIRprism 5300.

The projection apparatus 5010 employs a single spatial light modulator5100 for projecting a color display on the screen 5900 by applying asequential color display method. Specifically, the variable light source5210 comprises a red laser light source 5211, a green laser light source5212 and a blue laser light source 5213 (not specifically shown here).The variable light source allows independent controls for the lightemission states. The controller of the variable light source performs anoperation of dividing one frame of display data into a plurality ofsub-fields (i.e., three sub-fields, that is, red (R), green (G) and blue(B) in the present case) and turns on each of the red laser light source5211, green laser light source 5212 and blue laser light source 5213 toemit each respective light in time series at the time band correspondingto the sub-field of each color as will be described later. In anexemplary embodiment, the light sources are laser light sources. Inalternate embodiment, the light sources may also be semiconductor lightsources such as the light emitting diodes (LEDs).

FIG. 7 is a functional block diagram for showing the configuration of aprojection apparatus according to an alternate preferred embodiment ofthe present invention.

The projection apparatus 5020 is commonly referred to as amultiple-plate projection apparatus that includes a plurality of spatiallight modulators 5100 instead of a single SLM included in thesingle-panel projection apparatus 5010 described earlier. Furthermore,the projection apparatus 5020 comprises a control unit 5502 in place ofthe control unit 5500.

The projection apparatus 5020 comprises multiple spatial lightmodulators 5100 further includes a light separation/synthesis opticalsystem 5310 between the projection optical system 5400 and each of thespatial light modulators 5100.

The light separation/synthesis optical system 5310 comprises multipleTIR prisms, i.e., a TIR prism 5311, a prism 5312, and a prism 5313.

The TIR prism 5311 directs the illumination light 5600 incident from theside of the optical axis of the projection optical system 5400 to thespatial light modulator 5100 as incident light 5601.

The TIR prism 5311 carries out the function of directing theillumination light 5600 projected along the optical axis of theprojection optical system 5400 and directs the light to the spatiallight modulator 5100 as incident light 5601. The TIR prism 5312 carriesout the function of separating red (R) light from an incident light5601, projected by way of the TIR prism 5311, to transmit the red lightto the spatial light modulators for the red light 5100, and furthercarries out the function of directing the reflection light 5602 of thered light to the TIR prism 5311.

Likewise, the prism 5313 carries out the functions of separating blue(B) and green (G) lights from the incident light 5601 projected by wayof the TIR prism 5311, and directs the light to the blue color-usespatial light modulators 5100 and green color-use spatial lightmodulators 5100, and further carries out the function of directing thereflection light 5602 of the green light and blue light to the TIR prism5311.

Therefore, the spatial light modulations of these three colors, R, G andB carry out these functions simultaneously by these three spatial lightmodulators 5100. The reflection light 5602, resulting from therespective modulations, is projected onto the screen 5900 as theprojection light 5603 by way of the projection optical system 5400, andthus a color display is achieved.

Note that the system may implement various modifications by using alight separation/synthesis optical system instead of the lightseparation/synthesis optical system 5310 described above

FIG. 8A is a functional block diagram for illustrating the configurationof the control unit 5500 for the above described single-panel projectionapparatus 5010. The control unit 5500 comprises a frame memory 5520, anSLM controller 5530, a sequencer 5540, a light source control unit 5560,and a light source drive circuit 5570.

The sequencer 5540, includes a microprocessor to control the operationtiming of the entire control unit 5500 and the spatial light modulators5100.

In one exemplary embodiment, the frame memory 5520 retains one frame ofinput digital video data 5700 received from an external device (notshown in the figure) connected to a video signal input unit 5510. Theinput digital video data 5700 is updated in real time whenever thedisplay of one frame is completed.

The SLM controller 5530 processes the input digital video data 5700 fromthe frame memory 5520 as described later. The SLM controller separatesthe data read from the memory 5520 into a plurality of sub-fieldsaccording to detail descriptions further describe below. The SLMcontroller outputs the data subdivided into subfields to the spatiallight modulators 5100 as binary data 5704 and non-binary data 5705,which are used for implementing an the ON/OFF control and oscillationcontrol (which are described later) of a mirror 5112 of the spatiallight modulator 5100.

The sequencer 5540 outputs a timing signal to the spatial lightmodulators 5100 synchronously with the generation of the binary data5704 and non-binary data 5705 at the SLM controller 5530.

The video image analysis unit 5550 outputs a image analysis signal 5800used for generating various light source pulse patterns (which aredescribed later) corresponding to the input digital video data 5700inputted from the video signal input unit 5510.

The light source control unit 5560 controls the light source drivecircuit 5570 to control the operation of the variable light source 5210by using a light source profile control signal in emitting theillumination light 5600 The light source profile control signal isgenerated from the image analysis signal 5800 on the basis of the inputof the image analysis signal 5800 generated by the video image analysisunit 5550 using data of the light source pulse patterns generated by thesequencer 5540 as will be further described below.

The light source drive circuit 5570 drives the red laser light source5211, green laser light source 5212, and blue laser light source 5213 ofthe variable light source 5210 to emit light, respectively. The lightsource generates the light source pulse patterns 5801 through 5811(which are described later) received from the light source control unit5560.

FIG. 8B is a functional block diagram for illustrating the configurationof the control unit of a multi-panel projection apparatus according tothe present embodiment.

The control unit 5502 comprises a plurality of SLM controllers 5531,5532 and 5533, which are used for controlling each of the spatial lightmodulators 5100. Each of these modulators is implemented for modulatingthe respective colors R, G and B, and the configuration of thecontrollers is the main difference between the control unit 5502 and thecontrol unit 5500 described in FIG. 8A.

Specifically, each of the SLM controller 5531, SLM controller 5532 andSLM controller 5533, is implemented to process the modulation of arespective colors Red, Green, and Blue.

Furthermore, a system bus 5580 is used for connecting the frame memory5520, light source control unit 5560, sequencer 5540, and SLMcontrollers 5531 through 5533, in order to speed up and streamline theconnection path of each connecting element.

FIG. 9 is a functional block diagram for illustrating the configurationof the light source drive circuit 5570 (i.e., the light source drivecircuits 5571, 5572, and 5573) according to the present embodiment.

The light source drive circuit illustrated in FIG. 9 comprises multipleconstant current circuits 5570 a (i.e., I (R, G, B)₁ through I (R, G,B)_(n)) and multiple switching circuits 5570 b (i.e., switching circuitsSW (R, G, B)₁ through SW (R, G, B)_(n)), which correspond to theirrespective constant current circuits 5570 a, in order to generate thedesired light intensities of emission P₁ through P_(n), for the lightsource optical system 5200 (i.e., the red laser light source 5211, greenlaser light source 5212, and blue laser light source 5213).

The switching circuit 5570 b switches according to the desired emissionprofile of the light source optical system 5200 (i.e., the red laserlight source 5211, green laser light source 5212, and blue laser lightsource 5213).

The setup values of the output current of the constant current circuits5570 a (i.e., constant current circuits I (R, G, B)_(n)), when the grayscale of the emission intensity of the light source optical system 5200is designated at N bits (where N>n), are as follows:

I(R, G, B)₁ = I_(th) + LSB I(R, G, B)₂ = LSB + 1 I(R, G, B)₃ = LSB + 2 …… I(R, G, B)_(n) = MSB

This illustrates that a gray scale display based on emission intensity;a similar gray scale display is achievable even if the emission period(i.e., an emission pulse width), emission interval (i.e., an emissioncycle), and the like, are made to be variable.

The relationship between the emission intensity Pn of the variable lightsource and drive current for each color in this case is as follows. Notethat “k” is an emission efficiency corresponding to the drive current:

P₁ = k * (I_(th) + I₁) P₂ = k * (I_(th) + I₁ + I₂) … …P_(n) = k * (I_(th) + I₁ + I₂ + … + I_(n − 1) + I_(n))

FIG. 10 is a chart showing the relationship between the applied currentI and emission intensity P_(n) of the constant current circuit 5570 a ofthe light source drive circuit shown described in FIG. 9.

Note that the description for FIG. 9 illustrates a case of changing theemission profiles of the variable light source for each sub-framecorresponding to each gray scale bit. If the display gray scale functionof the spatial light modulator 5100 is used in parallel, the number ofrequired levels of electrical current decreases, enabling a reduction inthe numbers of constant current circuits 5570 a and switching circuits5570 b. This also makes it possible to obtain a number of gray scalesequal to, or higher than, the gray scales of spatial light modulator5100.

The following detail description explains the configuration of thespatial light modulator 5100 according to the present embodiment.

The spatial light modulator 5100 according to the present embodiment isa deflectable mirror device with an array of mirror elements. FIG. 11 isa schematic circuit diagram for illustrating the layout of the internalconfiguration of the spatial light modulator 5100 according to thepresent embodiment. FIG. 12 is a cross-sectional diagram of anindividual pixel unit implemented in the spatial light modulator 5100according to the present embodiment. FIG. 13 shows a side view diagramfor illustrating the configuration of individual pixel unit constitutingthe spatial light modulator 5100 according to the present embodiment.

FIG. 11 shows an exemplary embodiment of the mirror device 5100 thatincludes a mirror element array 5110, column drivers 5120, ROW linedecoders 5130, and an external interface unit 5140. The externalinterface unit 5140 includes a timing controller 5141 and a selector5142. The timing controller 5141 controls the ROW line decoder 5130 onthe basis of a timing signal from the SLM controller 5530. The selector5142 supplies the column driver 5120 with a digital signal from the SLMcontroller 5530. A plurality of mirror elements 4001 are arrayed as amirror element array 5110 at the positions aligned with individual bitlines. The bit lines are vertically extended from the column drivers5120, crosses individual word lines. The word lines are horizontallyextended from the row decoders 5130.

As shown in FIG. 12, the individual mirror element 5111 includes amirror 5112 supported on a substrate 5114 by a hinge 5113 to deflectwithin a range of deflection angles. The mirror 5112 is covered with acover glass 5150 for protection. The mirror is controlled to operate asan OFF electrode 5116 inclining to and contacting an OFF stopper 5116 a,and to operate as an ON electrode 5115 inclining to and contacting an ONstopper 5115 a. The ON and OFF angular positions are symmetricalrelative to the hinge 5113 on the substrate 5114.

A voltage applied to an OFF electrode 5116 draws the mirror 5112 with aCoulomb force to deflect toward the OFF stopper 5116 a. The mirror 5112reflects the incident light 5601 onto the light path in an OFF directionthat is offset from the optical axis of the projection optical system5400.

A voltage applied to the ON electrode 5115 draws the mirror 5112 with aCoulomb force to deflect toward the ON stopper 5115 a. The mirror 5112reflects the incident light 5601 onto the light path of in ON directionaligned with the optical axis of the projection optical system 5400.

Disclosure Content 4

The following detail description illustrates the preferred embodiment ofthe present invention with reference to the accompanying drawings.

The following description is related to alternate embodiments. Thedescription takes into account of the configurations and operations ofthe projection apparatuses illustrated in the above described FIGS. 6through 13. Note that the same alphanumeric designations are assigned tothe same component d the above descriptions.

As explained above, an image projection apparatus employs a spatiallight modulator 5100 implemented as a mirror device. According to thepresent embodiment, the mirror device is configured to perform a lineargray scale display that is different from a conventional displayapparatus such as CRT.

Therefore, FIG. 14, illustrates a γ correction, such as an input data γcurve 7700 a, is applied to a piece of input digital video data 5700 atthe transmission source (i.e., where the imaging is carried out).Assuming a display in the CRT, a projection apparatus comprising adisplay device other than the CRT is required to restore thecharacteristic of a gray scale display to the original state (e.g., aconversion line 7700L for performing a linear conversion of a brightnesssignal in terms of an input data signal). This is done by means of acorrection such as a γ correction curve 7700 b and/or to perform variousγ corrections in accordance with the characteristics of the projectionapparatuses 5010, and 5020.

In such a case, a mathematical operation for the input digital videodata 5700, as it is performed in a conventional display device, causesthe circuit scale of the control unit 5500 to increase, leading to ahigher production cost.

The present embodiment configured so that the above described videoimage analysis unit 5550 changes the emission pattern of theillumination light 5600 emitted from a variable light source 5210 to theprofile, as indicated by a γ correction light intensity variation 7800a, so as to follow the above noted γ correction curve 7700 b, asillustrated in FIG. 15. Thereby a linear gray scale display as indicatedby the conversion line 7700L is attained by negating the influence ofthe input data γ curve 7700 a performed at the transmission sourcewithout requiring a mathematical operation of the input digital videodata 5700.

Note that this configuration makes it possible not only to restore thelinearity by negating the influence of the input data γ curve 7700 a butalso to change, intentionally nonlinearly, the emission intensities ofthe variable light source 5210 within one frame as described below,thereby enabling various and highly precise gray scale displays inexcess of the original gray scale control capability of the spatiallight modulator 5100.

For example, a video image output (i.e., the input digital video data5700) contains various scenes such as a dark scene, a bright scene, agenerally bluish scene, and a generally reddish scene. The projectionapparatus according to the present embodiment is configured to controlthe gray scale of the emission output of the variable light source 5210optimally for each scene (with actual control carried out in units offrame), thereby making it possible to attain higher quality videoimages.

When a γ correction of the input digital video data 5700 (i.e., theinput data γ curve 7700 a) is implemented by means of a temporal changein emission intensities of the variable light source 5210 as describedabove, a precise emission control of the variable light source 5210 isdifficult if an ON/OFF control of the mirror 5112 is carried out througha pulse width modulation (PWM), which uses the binary data 7704 includedin the input digital video data 5700.

Thus, the SLM controller 5530 according to the present embodiment isconfigured to carry out an ON/OFF control of the mirror 5112 usingnon-binary data 7705 obtained by converting binary data 7704 asillustrated in FIGS. 16, 17, 18 and 19.

FIG. 16 illustrates 1) the generation of non-binary data 7705 (which isa bit string with each digit being of equal weight) from the binary data7704 comprising, for example, 8-bit “10101010”, and 2) the turning ON ofthe mirror 5112 only for the period in which the bit string continues.

Note that FIG. 16 illustrates 1) the conversion of non-binary data 7705so that the bit string is packed forward within the display period ofone frame, and 2) the turning ON of the mirror 5112 for a predeterminedperiod in accordance with the bit string number from the beginning of aframe display period.

Likewise, FIG. 17 illustrates the conversion of 8-bit “01011010” binarydata 7704 into non-binary data 7705, which is a forward-packed bitstring.

Furthermore, FIG. 18 illustrates the conversion of binary data 7704illustrated in the above described FIG. 16 into a bit string ofnon-binary data 7705, with the digits packed backward. In this case, themirror 5112 turned ON only in the period of time corresponding to thebit string number starting from the middle of a frame display perioduntil the end thereof.

Likewise, FIG. 19 illustrates 1) the conversion of binary data 7704illustrated in the above described FIG. 17 into a bit string ofnon-binary data 7705, with the digits packed backward, and 2) theturning ON/OFF of the mirror 5112.

When the ON/OFF is controlled by the non-binary data 7705 as describedabove, the ON period of the mirror 5112 becomes continuous, and,therefore, control of the emission intensity of the variable lightsource 5210 synchronous with the aforementioned ON period can be moreconveniently achieved.

FIG. 20 illustrates 1) the brightness input of 8-bit non-binary data7705 into, for example, four steps, i.e., 64, 128, 192 and 255, as shownin the upper rows of FIG. 20, and 2) obtaining a γ correction curve 7700c as shown in the lower row of the drawing through a four-step controlof the output intensity of the variable light source 5210 in response tothe each of the aforementioned levels, as indicated by a light sourcepulse pattern 7801 shown in the middle row of the drawing.

FIG. 20 illustrates the sequence of this control process in four steps.A further minute grouping of the non-binary data 7705 makes it possibleto obtain a smoother curve than the γ correction curve 7700 c.

Note that FIG. 20 shows that the correction amount of the γ correctioncurve 7700 c is less bright when compared with the conversion line7700L. Accordingly, the emission pattern of the variable light source5210 may be controlled so as to move a γ correction curve 7700 d closerto the above described conversion line 7700L (as indicated in the bottompart of FIG. 21). This is done by increasing the emission intensity ofthe light source pulse (from the emission intensity H0 to the emissionintensity H1) toward the tail end of the display period of one frame asindicated by a light source pulse pattern 7802, as illustrated in FIG.21.

FIGS. 20 and 21 illustrate a γ correction by changing the emissionintensity while maintaining the variable light source 5210 so that itcontinuously emits light, as indicated by the light source pulsepatterns 7801 and 7802. The control may be performed by means of anintermittent pulse emission.

FIG. 22 illustrates a control by means of the aforementionedintermittent pulse emission. A light source pulse pattern 7803illustrated in FIG. 22 generates emission pulses with a pulse width tp.This is done intermittently in emission pulse intervals ti and increasesthe number of emission pulses per unit time by gradually decreasing theemission pulse interval ti between the beginning and end of the displayperiod of one frame. This thereby achieves an effect similar to that ofthe above described continuous light source pulse patterns 7801 and7802. Furthermore, the light source pulse pattern 7804 illustrates thegradual increase of the emission pulse width tp between the beginningand end of the display period of one frame. Additionally, the lightsource pulse pattern 7805 illustrates the gradual decrease of theemission pulse intervals ti and also the gradual increase of theemission pulse width tp between the beginning and end of the displayperiod of one frame. Then, the light source pulse pattern 7806illustrates the gradual increase of both the emission pulse width tp andthe emission intensity H2 between the beginning and end of the displayperiod of one frame.

FIGS. 23A and 23B illustrate a γ correction curve 7700 e performing γcorrection to more effectively correct the lower brightness side bymeans of a light source pulse pattern 7807.

Therefore, the light source pulse pattern 7807 shown in FIG. 23Acontrols the emission pattern of the variable light source 5210 so as tocause the generation of multiple emission pulses. These have a constantemission pulse width tp (with a small emission pulse interval)) on thestart side (i.e., the lower density side) of the display period of oneframe. to the light source pulse pattern 7807 gradually decreases thenumber of pulses (that is, the emission pulse interval ti graduallyincreases) toward the end of the display period. The control processmakes it possible to attain a γ correction curve 7700 e, which isconvex-shaped toward the top-left of the conversion line 7700L, andwhich, accordingly, provides more effective correction, i.e., increasingbrightness, on the lower brightness side, as illustrated in FIG. 23B.

FIGS. 24A and 24B illustrate a γ correction with consideration given tohuman vision. This is done by a control of the variable light source5210 with a light source pulse pattern 7808. The human eye is highlysensitive to the mid-range of low and high brightness. Accordingly, a γcorrection is performed by controlling the variable light source 5210with the light source pulse pattern 7808, which 1) causes emissionpulses to have the same emission pulse width tp with a small emissionpulse interval ti at the center of the display period of one frame and2) gradually decreases the density of the emission pulse toward eitherside, as illustrated in FIG. 24A. This control achieves a γ correctionusing a γ correction curve 7700 f that is smaller than the conversionline 7700L on the lower brightness side and larger than that on thehigher brightness side, thereby making it possible to obtain a modulatedand clear projection image, i.e., darker on the lower brightness sideand brighter on higher brightness side, as shown in FIG. 24B. Also, whengamma correction by means of modulation in the light source asimplemented in the present embodiment is performed after a conventionalgamma correction with the above mathematical operation, by using a gammacorrection circuit, it is possible to perform a more effective gammacorrection to the mirror device than the conventional correction.

The following description discloses an example of expanding the numberof display gray scales by performing a modulation control of theaccumulated maximum light intensity in the display period of one frame,which corresponds to the variable light source 5210 of each color so asto obtain a desired output light intensity corresponding to the pixeldata indicating the maximum brightness. The maximum gray scale outputprovided by a spatial light modulator 5100 is determined by theoperation speed of the ON/OFF control of a mirror (more specifically,however, it is affected by other factors such as single-panel comprisalversus multi-panel comprisal and the number of sub-frames it is to bedivided into). For example, if an 8-bit gray scale output is the maximumaccording to the operation speed of the mirror 5112, a 256-step grayscale, i.e., “0” through “255”, can be the output. If a single colorgradation is displayed, the gradation is 256 steps; the gradationrecognition capability of humans exceeds 256 steps (it is believed that12-bit gray scale would suffice) so that the display does not present asmooth gradation to the human eye.

In practice, however, there are few scenes that output the entire grayscales of “0” through “255”, and, instead, there are more cases ofoutputs ranging only from “0” through “128”, such as in a movie, or evendarker gray scale images. Human visual recognition capability isparticularly equipped to detect differences in gray scale in dark areasthan in bright areas. Therefore a person tends to recognize as anoutline a minute difference in brightness in a dark image.

The present embodiment is accordingly configured to perform a modulationcontrol of an accumulated maximum light intensity in the display periodof one frame corresponding to the variable light source 5210 of eachcolor so that a desired output light intensity corresponding to themaximum brightness pixel data is obtained. This control makes itpossible to express the brightness from the brightest part (i.e., thepixel) to the total darkness (i.e., “0” brightness) of a scene (i.e.,frame) with the maximum gray scale output of a mirror, thereby smoothingand refining the video image, especially in a dark image.

The upper part of FIG. 25 shows 1) variable light sources 5210 (i.e.,the red laser light source 5211, the green laser light source 5212, andthe blue laser light source 5213) continuously emitting light at aconstant emission intensity H10 in the gray scale display control of therespective colors in, for example, the multi-panel projection apparatus5020 and 2) the turning ON/OFF of the mirrors 5112 in accordance withthe mirror control profile 7706 (for red), mirror control profile 7707(for green), and mirror control profile 7708 (for blue) by means of thePWM, and thereby achieving a gray scale display.

When controlling a gray scale by means of the ON/OFF control of themirror 5112, a smooth gray scale cannot be expressed because theexpression depends on the gray scale expression of the data width of theinput digital video data 5700. Furthermore, the light sources of therespective colors have a constant emission state irrespective of changesin the gray scale, thereby wasting the emission energy.

In contrast, the present embodiment maintains the mirror 5112 of apixel, which indicates the maximum brightness, continuously at the ONstate (in accordance with the mirror control profiles 7706 a, 7707 a and7708 a) and sets the variable light sources 5210 (i.e., red laser lightsource 5211, green laser light source 5212, and blue laser light source5213), which output the illumination light 5600, at emission intensitiesH11 (for red), H12 (for green), and H13 (for blue). These correspond tothe gray scale data indicating the maximum brightness, and the grayscale control of each color illustrated on the lower side of FIG. 25.Therefore, the gray scale can be expressed by the maximum gray scaleoutput (that is, a continuous ON state in one frame period) of themirror 5112, thereby smoothing out and refining the video image,especially in a dark scene.

Furthermore, the brightness of the colors R, G and B is attained by theincrease/decrease in the intensity of the illumination light 5600 outputfrom the corresponding variable light sources 5210 (i.e., the red laserlight source 5211, the green laser light source 5212, and the blue laserlight source 5213), thereby saving energy, reducing an unnecessary lightcomponent, and improving the contrast in the video image.

Note that while the above described FIG. 25 illustrates the variablelight sources 5210 as continuously turned on at the emission intensitiesH11, H12, and H13, in the gray scale control of the respective colors,the variable light sources 5210 may be controlled with an intermittentemission pulse, as shown in FIG. 26.

That is, in FIG. 26, the mirror 5112 of a pixel indicating the maximumbrightness is maintained at a continuous ON state in one frame period,as represented by the mirror control profiles 7706 a, 7707 a and 7708 ain the display control of the respective colors. The variable lightsources 5210 are configured to pulse-emit in accordance with theemission pulse width tp and emission pulse interval ti, as representedby light source pulse patterns 7809 b (for red), 7810 b (for green), and7811 b (for blue). In this event, the number of emission pulses iscontrolled so that the total intensity of the emission pulse isequivalent to the gray scale data of a pixel indicating the maximumbrightness. Also in this event, the gray scale can be expressed by themaximum gray scale output (that is, a continuous ON state in one frameperiod) of the mirror 5112, thereby smoothing out and refining the videoimage, especially in a dark scene. Furthermore, the brightness of thecolors R, G and B are attained by the increase/decrease in the intensityof the corresponding variable light sources 5210 (i.e., red laser lightsource 5211, green laser light source 5212, and blue laser light source5213), which saves energy and reduces an unnecessary light component,and improves the contrast in the video image.

FIG. 27 illustrates a gray scale control when the gray scale controlillustrated in FIGS. 25 and 26 is applied to a single-panel projectionapparatus 5010. In this case, the display period of one frame is dividedinto multiple subfields 5701, 5702, and 5703 corresponding to the colorsR, G, and B, respectively, and a color display is attained by a colorsequence method. However, with the conventional method, the ON/OFFcontrol for the mirror 5112 is performed, by means of a PWM, inaccordance with the mirror control profiles 7706 (for red), 7707 (forgreen), and 7708 (for blue) in the respective subfields, and thevariable light sources 5210 perform a continuous emission at a constantintensity level in accordance with the light source pulse patterns 7809,7810 and 7811, thereby performing a gray scale control, as shown on theupper part of FIG. 27. In this case, a gray scale expression depends onthat of the data width of input digital video data 5700 and therefore asmooth gray scale expression cannot be attained as described above.

In contrast, as shown on the lower part of FIG. 27, the presentembodiment is configured to perform a control so that the mirror 5112 ofa pixel indicating the maximum brightness is controlled to the ON statein the entire display period of one frame (i.e., the entire subfields)in accordance with the mirror control profiles 7706 a, 7707 a, and 7708a, and so that the intensity of the variable light sources 5210 are setat an intensity equivalent to the gray scale data of a pixel indicatingthe maximum brightness (i.e., the emission intensities H11 (for red),H12 (for green) and H13 (for blue)). Therefore, the gray scale can beexpressed by the maximum gray scale output (that is, a continuous ONstate during the period of one frame) of the mirror 5112, thus smoothingout and refining the video image, especially in a dark scene.

FIG. 28 shows the attainment of an intensity equivalent to the abovedescribed emission intensities H11, H12, and H13 by adjusting theemission pulse width tp and emission pulse interval ti of the emissionpulse by means of an intermittent pulse emission of the variable lightsources 5210 in the respective subfields of red, green, and blue. Thusan effect similar to the case described FIG. 27 is obtained.

FIG. 29 shows the capability of a grayscale control with a wider dynamicrange than the emission intensity of the variable light source 5210 bycombining the ON/OFF control of the mirror 5112 and the emissionintensity control of the variable light source 5210 in theabove-described examples. Accordingly, if the emission intensity levelof the variable light source 5210 is constant at the emission intensityH20, with a gray scale expression in 256 steps, that is, “0” through“255”, in accordance with input digital video data 5700 being possibleonly in the range between the full ON and full OFF of the mirror 5112,and a pixel indicates the maximum brightness as half the lightintensity, i.e., “0” through “127”, then only a 128-step gray scale,i.e., “0” through “127”, can be expressed, as shown on the upper part ofFIG. 29.

In contrast, if the emission intensity of the variable light source 5210is controlled to maintain the emission intensity H21 of the variablelight source 5210 at one half of the emission intensity H20, as in thepresent embodiment, then it is possible to attain a 256-step grayscaleexpression, i.e., “0” through “255”, in the range between the full ONand full OFF states of the mirror 5112, as shown on the lower part ofFIG. 29.

For this reason, the range of the grayscale expression can exceed thedesignation range of the input digital video data 5700, thus improvingthe image quality.

The following description explains countermeasures for prevent theproblem of a color break. In the case of a multi-panel projectionapparatus comprising multiple spatial light modulators 5100, as in theabove described projection apparatus 5020, there is a concern that, ifthe output time for each color is different, a state in which only acertain color is outputted is created, resulting in the occurrence of acolor break, in which the individual colors R, G, and B are seenseparately. Accordingly, the present embodiment is configured to equipthe SLM controller 5530 to control the mirror 5112 of the spatial lightmodulators 5100 so that during either the transition between the ON andOFF states, or the intermediate output state, the mirror 5112 oscillatesbetween the ON and OFF states. Furthermore, if the brightness outputvalue to be modulated is no smaller than the brightness output of theintermediate output state during the entire display period of one framefor each color, the modulation is performed between the ON state andintermediate output state of the mirror 5112 for the display period ofone frame for each color.

FIG. 30 illustrates a countermeasure for a color break. The mirrorcontrol profile 7711 shown at the center of FIG. 30 indicates abrightness output carrying out a mirror oscillation control 7710 bduring the entire display period of one frame for each color.Furthermore, the present embodiment is configured to continue to outputlight during the entire display period of one frame through acombination of a mirror ON/OFF control 7710 a and a mirror oscillationcontrol 7710 b as indicated by the mirror control profile 7710 on thetop side of FIG. 30, wherein the brightness output is no less than themirror control profile 7711. In contrast, when the brightness output isno more than the mirror control profile 7711, the necessary brightnessoutput is attained by controlling a continuation time period of themirror oscillation control 7710 b during the display period of oneframe, as shown on the lower side of FIG. 30.

The control illustrated in FIG. 30 makes it easy to align the outputtime for each color, thereby reducing occurrence of a color break in theprojection apparatus 5020 comprising multiple spatial light modulators5100. Note that, if a grayscale control is carried out by controllingthe intensity through setting the emission pulse width tp and emissionpulse interval ti of the variable light source 5210, as in the abovedescribed FIGS. 26, 28, etc., the light source control unit 5560 is alsocapable of increasing the maximum brightness of the variable lightsource 5210. This is done by selectively narrowing the emission pulseinterval ti within a specific time during a one-frame period, for aframe of a specific condition of the input digital video data 5700, whenthe output of the illumination light 5600 is modulated by varying theemission pulse interval ti (i.e., the emission interval cycle) of thepulse emission of the variable light source 5210.

This takes advantage of the maximum brightness of the variable lightsource 5210 by widening the dynamic range of a video image output,thereby making it possible to obtain a more powerful video image.Therefore, the configuration increases the maximum brightness of thevariable light source 5210 by putting it in over-drive only whendisplaying a scene (i.e., a frame) in which, for example, only a smallpart of a screen is very bright, or the like, as described above. Thisis particularly advantageous it avoids a continuous setup of the maximumbrightness, which adversely affects the durability of the variable lightsource 5210.

Note that the present invention can be modified in various ways withinthe scope of the configurations illustrated in the above-describedpreferred embodiments.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alternationsand modifications will no doubt become apparent to those skilled in theart after reading the above disclosure. The present invention may bechanged in various manners possible within the scope of the presentinvention, and is not limited to the configurations exemplified in theabove-described embodiments. Accordingly, it is intended that theappended claims be interpreted as covering all alternations andmodifications as fall within the true spirit and scope of the invention.

1. A projection apparatus, comprising: a light source; a light sourcecontrol unit for controlling output of the light source; at least onespatial light modulator for modulating illumination light from the lightsource by multiple pixel elements; and an optical system for projecting,onto a screen, the illumination light deflected by the spatial lightmodulator, wherein the light source control unit modulates the output ofthe illumination light from the light source during a modulation periodof the spatial light modulator, and non-linearly controls gray scale ofan image projected onto the screen.
 2. The projection apparatusaccording to claim 1, wherein: the light source control unit modulatesoutput of the illumination light by making the intensity of theillumination light variable.
 3. The projection apparatus according toclaim 1, wherein: the light source control unit modulates output of theillumination light by making the emission interval cycle of the pulseemission of the light source variable.
 4. The projection apparatusaccording to claim 1, wherein: the light source control unit modulatesoutput of the illumination light by making the emission interval cycleof that pulse emission of the light source constant and by making theemission pulse time variable.
 5. The projection apparatus according toclaim 1, wherein: the light source control unit modulates output of theillumination light by making the emission interval cycle and theemission pulse time of pulse emission of the light source variable. 6.The projection apparatus according to claim 1, wherein: the light sourcecontrol unit modulates output of the illumination light by making atleast one of the emission interval cycles, the emission pulse time, orthe emission pulse intensity of pulse emission of the light sourcevariable.
 7. The projection apparatus according to claim 1, wherein: thelight source control unit modulates output of the illumination light byinput image data.
 8. The projection apparatus according to claim 1,wherein: the light source control unit increases the maximum brightnessof the light source by making the emission interval cycle of the pulseemission variable within a particular time of one frame, when output ofthe illumination light is modulated by making the emission intervalcycle of the pulse emission of the light source variable.
 9. Theprojection apparatus according to claim 1, wherein: the spatial lightmodulator comprises a micromirror device in which multiple mirrorelements for deflecting light from the light source are arranged. 10.The projection apparatus according to claim 1, wherein: the light sourceis a light emitting diode (LED) or a laser device.